A laser diode bar refers to a semiconductor light resource in which emitters corresponding to diode lasers are arranged on one-dimensional array, and frequently used as a pumping light resource of a solid state laser or an optical fiber laser.
FIG. 1 illustrates types of beams emitted from a laser diode bar.
Referring to FIG. 1, a radiation angle 112 perpendicular to an emitter 100, that is, with respect to a fast axis is very large ranging from 30 to 40 degrees, but a radiation angle 114 parallel to the emitter 100, that is, with respect to a slow axis is very small ranging from 6 to 10 degrees. Here, in order to collimate a fast axis beam having a large radiation angle, a cylindrical lens having a high numerical aperture (hereinafter, referred to as an “NA”) should be used. When a width 122 of a single emitter is narrower, beam quality is more improved, and when an interval 124 between emitters is wider, heat is more easily emitted. Accordingly, a space may exist where light is not emitted between the emitters, thereby deteriorating the beam quality.
In U.S. Pat. No. 6,384,981 entitled “Optical emitter array with collimating optics unit”, D. Hauschild proposes a method of removing an empty space between emitters and increasing an optical fill factor by enlarging a width of the emitter by an interval between the emitters with the use of two micro-optic cylindrical lens arrays.
In the paper entitled “Beam collimation of high-power laser diode array with graded-index fiber lens array”, Optical Engineering 47(5), 054202, 2008, Y. Tang et al. describes a method of making beams of respective emitters symmetrical to each other by using a graded-index fiber lens array as a slow axis collimator and collimating the beams. The collimated beams may be combined with a multi-mode optical fiber. Such a method has advantages corresponding to low costs and high yields, but has disadvantages in that the method cannot be used for an emitter interval narrower than a diameter of a graded-index optical fiber and beam quality is deteriorated.
Optical power of the laser diode bar is frequently used in combination with the optical fiber in order to be used for pumping the optical fiber laser or use circular beam quality and manageable characteristics of the optical fiber. In this case, asymmetrical beam quality between the fast axis and the slow axis of the laser diode bar is the problem. Beam quality of the light source is indicated by a beam parameter product (BPP), and expressed by a multiple of a beam waist corresponding to a beam size and a radiation angle. In a single emitter having a width of 100 μm, while the BPP of the fast axis is 0.3 mm*mrad, the BPP of the slow axis is 5 mm*mrad. The BPP of a representative input optical fiber for combination with the optical fiber having a core diameter of 105 μm and an NA of 0.15 is 8 mm*mrad.
FIG. 2 illustrates types of beams of the optical fiber.
When a long rectangular beam 210 of the single emitter is received using the optical fiber, many empty spaces are generated in a fast axis direction as illustrated in FIG. 2A. The above problem may be solved by using a heat sink in a step structure proposed in U.S. Pat. No. 7,848,372 entitled “Modular Laser Assembly” by D. Schulte. When single emitters are disposed on steps having slightly different heights in the fast axis direction, a sum of beams of the single emitters has a square shape as illustrated in FIG. 2B, thereby increasing an optical fill factor of the optical fiber. However, such a method has disadvantages in that it requires a very complex processing when the heat sink in the step structure is manufactured and cooling water cannot be directly flowed to the heat sink.
In the general laser diode bar, the BPP in the slow axis is equal to or larger than 200 mm*mrad even though the optical fill factor is increased by performing collimation in the slow axis. Since the BPP cannot be reduced in an optical system, a core diameter or an NA is larger than an optical fiber having a core diameter of 105 μm and an NA of 0.15 in order to receive all beams of the slow axis of a the single emitter, so that an optical fiber having a large BPP should be used. In U.S. Pat. No. 6,462,883 entitled “Optical Coupling Systems”, Z. Wang et al. proposes a method of re-arranging beams emitted from respective emitters on the fast axis by a laser diode bar. That is, by giving some angles to an axis of a cylindrical lens collimated in the fast axis direction with respect to an axis of the emitter, a slow axis beam of the LD bar has a different position in the fast axis direction. Further, beams divided using the optical system are imaged and arranged on the fast axis. In this case, the beams face different directions, and may move to an equal direction by using several mirrors and prisms. With the use of such a method, a beam having a rectangular shape with long side in a direction of the slow axis is divided into smaller beams, the divided beams are arranged on the fast axis, and a sum of the beams becomes a square shape, thereby increasing efficiency of combination with a circular optical fiber. However, the method has to use several optical components and requires a precise arrangement thereof.